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Materials Science and Engineering B 133 (2006) 70–76

dc and ac transport properties of Mn-doped ZnO thin filmsgrown by pulsed laser ablation

Dhananjay a,∗, J. Nagaraju a, S.B. Krupanidhi b

a Department of Instrumentation, Indian Institute of Science, Bangalore 560012, Indiab Materials Research Center, Indian Institute of Science, Bangalore 560012, India

Received 20 October 2005; received in revised form 10 May 2006; accepted 13 May 2006

bstract

MnxZn1−xO (x = 0.20) thin films were deposited on Pt coated Si substrates using pulsed laser ablation technique. The structural characteristicsf the films were investigated by X-ray diffraction (XRD), while the dielectric response of the films was studied as a function of frequency andmbient temperature by employing impedance spectroscopy. It was found that all the films deposited on Pt coated Si substrates had c-axis preferredrientation perpendicular to the substrate, with full width at half maximum (FWHM) of the (0 0 2) X-ray reflection line being less than 0.5◦. Thec and ac electrical conductivity of Mn-doped ZnO films were investigated as a function of temperature. The ac conductivity, σac(ω), varies asac(ω) = Aωs with s in the range 0.4–0.9. The complex impedance plot showed data points lying on a single semicircle, implying the responseriginated from a single capacitive element corresponding to the bulk grains. The value of the activation energy computed from the Arrhenius plot

f both dc and ac conductivities with 1000/T were 0.2 eV suggesting hopping conduction mechanism. The optical properties of Zn0.8Mn0.2O thinlms were studied in the wavelength range 300–900 nm. The data were analyzed in the light of the existing theories and reflected a Burstein–Mosshift in these films. The films show magnetic properties, which are best described by a Curie–Weiss type behavior.

2006 Elsevier B.V. All rights reserved.

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eywords: Impedance spectroscopy; Hopping conduction; Burstein–Moss shif

. Introduction

In recent years, spintronics [1,2] has been studied extensivelyy many of the researchers due to its spin-dependent phenom-na, which can be applied to modern electronic devices. Dilutedagnetic semiconductors (DMS) are considered ideal systems

or spintronics, in which transition metal ions replace cations ofost semiconductor materials. The replaced transition metal ionsouple with the electrons in the semiconducting band. Such apin–spin exchange interaction between the d electrons of mag-etic ions and band carriers results in large Zeeman splitting ofand states and giant Faraday rotation. Such a coupling leadso various interesting properties such as magneto-optical and

agneto-electrical effects [3]. ZnO can be used as a host mate-

ial for the growth of diluted magnetic semiconductors in which

n will substitute the Zn cations. Fukumura et al. [4] have stud-ed epitaxial thin films of Mn-doped ZnO by pulsed laser ablation

∗ Corresponding author.E-mail address: dhaya@isu.iisc.ernet.in ( Dhananjay).

twisocZ

921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2006.05.005

ith Mn content as high as 35 at.%. Because ZnO is an attrac-ive optical material with large optical band gap (∼3.3 eV) [5]nd high exciton binding energy (60 meV) [6], magneto-opticalroperties of DMS based on ZnO would be useful for a vari-ty of short wavelength optical applications. Though intensivefforts are evident on the growth of Mn-doped ZnO films byulsed laser deposition method, their electrical properties wereot studied in detail.

Impedance spectroscopy has been used as a powerful tech-ique to determine the conductivity of the ionic conductors [7].ecently, power law analysis of the ac conductivity data has been

ound to be a tool for studying the ion dynamics of such systems.n most of the doped semiconductors [8] transport mechanismccurs through hopping. As suggested by Pollak and Geballe [9],he transport occurs by the electrons hopping between states,hich are essentially localized around the acceptor or donor

mpurities. For such a hopping to take place, it is necessary for

ome of the localized states to be vacant and hence compensationf the majority impurity becomes essential feature of impurityonduction. In this paper, we report the growth of Mn-dopednO thin films by pulsed laser ablation. Hopping conduction in

ce and Engineering B 133 (2006) 70–76 71

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3

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3

aXwith increase in Ts. This would make the mobility to increase,which in turn increases the conductivity. In order to investigatethe nature of conductivity in these films temperature-dependentstudies were carried out. Hopping conduction in doped semicon-

Dhananjay et al. / Materials Scien

n-doped ZnO were studied and verified by analyzing the dcnd ac conductivity data.

. Experimental details

Pulsed laser ablation has been used to deposit thin films ofariety of materials including ZnO, due to several advantagesuch as good stoichiometry control, faster growth rate, etc. Aingle phase dense Zn1−xMnxO (x = 0.2) target was preparedia a conventional solid-state reaction by mixing and calcin-ng the multicomponent oxides at 900 ◦C for 4 h and then theowders were pressed into a pellet. The pressed pellet was sin-ered at 1000 ◦C for 4 h to form Zn1−xMnxO target. The targetas used for the ablation and the thin films were deposited onifferent substrates such as Pt coated Si and corning glass byrF (248 nm) pulsed excimer laser. The oxygen pressure in theeposition chamber was kept at 50 mTorr. Substrate tempera-ure was varied from 500 to 650 ◦C. The deposition was carriedut for 20 min. Structural phase determination was carried outsing X-ray diffractometer and the grain morphology was exam-ned by scanning electron microscope. The thickness of the filmas measured using Dektak thickness profilometer. The mea-

ured thickness of the films was about 0.2 �m. Nickel dots of.96 × 10−3 cm2 area were deposited by thermal evaporationnto Zn0.8Mn0.2O films as top electrodes in order to study thelectrical properties of the films. The electrodes were annealedt 200 ◦C for 30 min in order to minimize the contact resistancend to enhance the adhesion of the deposited metal film. Whilehe dc conductivity measurements were carried out using Van derauw technique, the ac conductivity measurements were donesing impedance spectroscopy. Capacitance–voltage character-stics were carried out using computer aided LCZ meter. Elec-rical properties in frequency domain (ac electrical properties)ere studied using Keithley 3330 LCZ meter with an oscilla-

ion level of 0.1 V over a temperature range of 30–100 ◦C. Theange of frequencies selected was from 100 Hz to 100 kHz. Mag-etic properties were examined using superconducting quantumnterference device magnetometer (SQUID).

. Results and discussion

.1. Structural characterization

The structure details of Mn-doped ZnO thin films were deter-ined by X-ray diffraction technique. Fig. 1 shows the XRD

attern of the films deposited at 50 mTorr at various substrateemperatures on Pt coated Si substrates. For the whole range ofeposition temperatures employed in this study (500–650 ◦C),ll the films were found to be c-axis oriented, exhibiting (0 0 2)RD reflection lines. Fig. 1 (inset) shows the FWHM of Mn-oped ZnO (0 0 2) XRD peak at different substrate temperatures.WHM was found to decrease with increase in substrate tem-erature, which was attributed to the increase in grain size. The

rain size of the films calculated from the Scherrer formula wasound to lie in the range of 30–40 nm. From the compositionalnalysis (obtained by EDAX) of the films, it was found that0 at.% is substituted in the ZnO lattice by Mn.

Fig. 1. XRD pattern of Zn0.8Mn0.2O films at different growth temperatures.

.2. dc conductivity

Fig. 2 shows the room-temperature electrical conductivity asfunction of substrate temperature (Ts). As calculated from theRD spectrum by Scherrer formula, the grain size increases

Fig. 2. Dependence of dc conductivity on substrate temperature (Ts).

72 Dhananjay et al. / Materials Science and Engineering B 133 (2006) 70–76

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ig. 3. Capacitance–voltage characteristics of Ni/Zn0.8Mn0.2O/Pt structure.

uctors depend on the doping concentration and the measure-ent temperature. In order to evaluate the doping concentration,

apacitance–voltage (C–V) measurements were carried out atoom temperature. Results of the C–V measurements are shownn Fig. 3. The doping concentration (Nd) and the barrier heightere determined from the C–V measurements, using the rela-

ion:

1

C2 = 2

A2

(Vbi − V

qεsNd

)(1)

here C is the measured capacitance, V the applied voltage,bi the built-in voltage, εs the dielectric constant, Nd the dop-

ng concentration and A is the area of the sample. The valuef Nd, obtained by fitting the data of Fig. 3 to Eq. (1) wereetermined to be 6.5 × 1016 cm−3. It was found that hoppingonduction [10,11] occurs in wide range of materials such asoped semiconductors, amorphous semiconductors, etc. For aoped semiconductor (e.g. Mn-doped ZnO), at low tempera-ures, most of the free electrons are recaptured by the donors. Asresult, electron hopping directly between donors in the impu-

ity band will make the main contribution to the conductivity.c conductivity of a material considering hopping mechanisms given by the relation [12]:

dc = CT−1 exp

(−ε)

(2)

kT

here C is a constant independent of temperature T, k the Boltz-ann constant and ε is the activation energy for the hops. We

nalyze our measurements obtained from the current–voltage

[

σ

Fig. 4. Arrhenius plot of the dc conductivity of Zn0.8Mn0.2O films.

haracteristics with variation in ambient temperature in theirespective Arrhenius plots. Fig. 4 shows the plot of ln(σdcT)ersus 1/T for Mn-doped ZnO films. Here T is taken as absoluteemperature for each measurement. All the data points trace atraight line and the activation energy calculated from the Arrhe-ius plot was found to be 0.23 eV and it is the energy required toop a charge carrier between the impurity sites. Further exper-mental support for the occurrence of hopping conduction in

n-doped ZnO thin films arises from the ac conductivity data,hich will be discussed in the next section.

.3. Dielectric studies

.3.1. ac conductivityTo further elucidate our results based on the dc transport char-

cteristics, we investigated the ac response of the samples toifferentiate as well as to identify the origin of the conductionrocess. The total conductivity σtot of any material is the sum-ation of dc conductivity σdc and ac conductivity σac:

tot = σdc + σac(ω) (3)

he σdc is independent of frequency and is due to free chargearriers in the system. The ac conductivity originates from theound and free charges and can be expressed in terms of thebsolute permittivity ε0 and the dissipation factor tan δ as

ac = ωε0ε′ tan δ (4)

here ω is the angular frequency and tan δ is the dissipationactor.

The frequency dependence of σac is given by the power law13]:

ac = An(T )[sin(n − 1)

π

2

]ωs (5)

Dhananjay et al. / Materials Science and Engineering B 133 (2006) 70–76 73

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lcTmaues are also in good agreement with the earlier reported values[16].

Fig. 5. ac conductivity of Zn0.8Mn0.2O films at different temperatures.

here s is a frequency-dependent parameter and represents theany body interactions of the electrons, charges and impuri-

ies. The power law nature of the ac conductivity correspondso the short range hopping of charge carriers through trap siteseparated by energy barriers of varied heights. The power lawehavior was directly reflected in the ac conductivity plot andig. 5 shows the ac conductivity as a function of frequency atarious temperatures on a Mn-doped ZnO film. The ac conduc-ivity obeys the power law relation; and at low frequencies, the aconductivity is strongly dependent on the temperature. However,t higher frequencies, the temperature dependence was foundeak. It is found that the exponent s shows a decreasing trendith increasing temperature. The exponent s for laser ablatedn-doped ZnO films is 0.9 at room temperature and showeddecreasing trend with increasing temperature (Fig. 6). Suchbehavior has been previously predicted by hopping over the

arrier (HOB) model [14,15]. This decreasing trend in s cane explained as follows: as the temperature increases, contribu-ion from the deep traps increases leading to the possibility ofand-to-band transition. Such transition tends to dominate theonduction process. As a result, s tends to zero since electroniconduction is frequency independent. Therefore it is apparenthat in the low temperature regime, the ac conductivity dependsignificantly on the frequency. And as the temperature increases,c conductivity takes over. Fig. 7 renders the plot of ln σacersus (1000/T) measured at a frequency of 50 kHz. The acti-ation energy computed from the plot is much less than half ofhe band gap, indicating extrinsic conduction. The frequency-ependent conductivity shows that the dominant conduction

echanism is hopping of charge carriers and the calculated acti-

ation energy of 0.2 eV could be associated with the energyequired for charge carriers to be released from a set of shal-

Fa

Fig. 6. Variation of the exponent s with temperature.

ow traps in Mn-doped ZnO films. This value found to be quitelose and consistent with that obtained in the dc measurements.he excellent agreement of the results from both dc and aceasurements reveals that the conduction process related to the

ctivation energy arises from similar sources. Further, these val-

ig. 7. Arrhenius plot of the ac conductivity of Zn0.8Mn0.2O films measured atfrequency of 50 kHz.

74 Dhananjay et al. / Materials Science and Engineering B 133 (2006) 70–76

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lretthe complex impedance plots had shown a decreasing trend withincrease in temperature and shown in Fig. 10 and were found tolie in the range 1.5 × 10−4 to 4.92 × 10−5 s. Since grain bound-aries are highly resistive by the virtue of the depletion layer

Fig. 8. Frequency dependence of Z′′ at different temperatures.

.3.2. Impedance spectroscopyTo study the transport behavior of the films in the vicinity of

he grain, in an applied alternating field, impedance spectroscopyas employed. Complex impedance spectroscopy is one of the

echniques for investigating dielectric materials [17,18]. Theontribution of various processes such as the electrode effects,ulk effects, and their interfaces namely grain boundaries cane resolved in the frequency domain. Complex impedance planelots of Z′ versus Z′′ (where Z′ and Z′′ are the real and imagi-ary parts of the complex impedance, respectively) are useful foretermining the dominant resistance of the sample. The complexmpedance Z* can be expressed in the following way:

∗ = R1 − jωτ

1 + ω2τ2 (6)

here R is the resistance of the parallel RC circuit, ω the angularrequency and τ is the relaxation time.

Fig. 8 renders the variation of Z′′ with frequency at temper-tures between 30 and 90 ◦C. The plots found to exhibit peakst different temperatures and peak maxima shifted to the highrequency side with increase in temperature. It is also foundhat the magnitude of imaginary component of the impedance athe peak frequency is a strong function of temperature indicatingrrhenius type behavior and from the Arrhenius plot of the peak

requency, the corresponding activation energy was calculatedo be 0.2 eV, which is in agreement with the values calculatedrom the ac conductivity measurements. In most of the ionicnd mixed ionic and covalent solid materials such as ZnO, theajor mode of charge transport is a multiple hopping process.

he hopping process normally takes place across the potentialarriers set up by the lattice structure and the local environmentf other atoms/ions. Fig. 9 shows the complex impedance plot ofhe films at different temperatures. It shows that the data points

Fig. 9. Arrhenius plot of the peak frequency.

ie on the single semicircle where centers lie on an axis off theeal axis. It is known that at the peaks of the semicircles, thequation ωτ = 1 holds, where τ is the relaxation time and ω ishe angular frequency. The relaxation times were computed from

Fig. 10. Cole–Cole plot of Zn0.8Mn0.2O films at different temperatures.

Dhananjay et al. / Materials Science and Engineering B 133 (2006) 70–76 75

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Fig. 11. Transmission spectra of Zn0.8Mn0.2O films.

19], the mean relaxation time of the grain boundaries is veryigh compared to the grain interiors. Therefore, the above resultuggests that the dielectric response originates from the inte-ior grains. In order to further confirm it, a careful investigationf the expression for the ac conductivity was carried out and iteveals that the conductivity is a product of the frequency andhe imaginary part of the dielectric constant and, therefore, whenhe slope of the log σ versus log ω curve is greater than unity,eflects an increase of ε′′ with the frequency. This indicates aoss peak in the ε′′ versus frequency curve. This loss peak cor-esponds to the crossover from the grain contribution to grainoundary contribution [20]. However, we did not observe suchoss peak over the entire temperature range indicating that theielectric response might originate from the grain interiors.

.4. Optical properties

The optical transmittance of the Zn0.8Mn0.2O thin films wasetermined by spectrophotometer within the wavelength range00–900 nm. For the transmission measurements, the films wererown on corning glass and irradiated at a perpendicular angle ofncidence with corning glass as the reference. Fig. 11 shows theransmission spectra of the films deposited at substrate temper-tures 400–600 ◦C. The average transmission in the wavelengthange 500–1000 nm is about 70%. The optical absorption coef-cient (α) is evaluated from the transmission spectra using theelation:

= 1

dln

(− 1

T

)(7)

here d is the thickness of the film and T is the transmittance.he optical band gap was evaluated using the relation:

αhυ)2 = A(hυ − Eg) (8)

attc

Fig. 12. Absorption spectra of Zn0.8Mn0.2O films.

here A is a constant, hυ the photon energy, and Eg is the energyap. Fig. 12 presents the dependence of the absorption coeffi-ient as a function of photon energy for Mn-doped ZnO films.he energy gap, Eg, estimated from the intercept of the lin-ar portion of the curve found to be 3.5 eV. A blue shift of thebsorption edge was observed with increase in substrate tem-erature as shown in Fig. 12. This could mainly be attributed tohe Burstein–Moss effect, since the absorption edge of a dopedemiconductor shifts to shorter wavelengths with increase inarrier concentration.

.5. Magnetic properties

We have investigated the magnetic properties of Mn-dopednO thin films grown by pulsed laser ablation. Magnetic proper-

ies were studied using a superconducting quantum interferenceevice (SQUID) magnetometer. The type of magnetic couplingn this system remains to be an issue of debate. In Fig. 13 weresent temperature dependency of magnetic susceptibility forhe sample with nominal composition Zn0.8Mn0.2O. Assuminghat the Mn ions interact with each other through the Heisenbergxchange, the susceptibility, χ, in the high temperature range cane expressed as

M

H= C(χ)

T − θ(χ)(9)

here C(χ) and θ(χ) are the Curie constant and Curie–Weissemperature, respectively.

It is clear from the Curie–Weiss plot (inset, Fig. 13) that a lin-ar behavior is observed at elevated temperature. A slight devi-

tion from the linear behavior at low temperature is attributed tohe anitferromagnetic Mn–Mn exchange interactions. Moreover,he large negative value of Curie–Weiss temperature (∼70 K)onfirms the strong anitferromagnetic exchange coupling in

76 Dhananjay et al. / Materials Science and

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ig. 13. Magnetization vs. temperature curve of Zn0.8Mn0.2O thin films (inset:urie–Weiss plot).

n1−xMnxO thin films. Therefore, it appears that Zn1−xMnxOhin films will not be useful for spintronics, unless additional car-iers are introduced in the lattice by some means as suggestedy Dietl et al. [21]. Efforts are in progress in that direction.

. Conclusions

In conclusion, in situ crystalline Mn-doped ZnO thin filmsere grown on Pt coated Si substrate by pulsed laser ablation

echnique. Highly (0 0 2) oriented films were grown with FWHMess than 0.5◦. We have analyzed the dielectric and ac conduction

ehavior of the polycrystalline Zn0.8Mn0.2O thin films. The dcnd ac conduction properties exhibited an activation energy of.2 eV and this was attributed to the shallow traps, which partic-pate in the charge carrier transport mechanism. The ac conduc-

[[

[

Engineering B 133 (2006) 70–76

ivity plot showed universal power law dependence accordingo the Jonscher model. The complex impedance plots exhibitedingle semicircle, which is a contribution from the grains. Theverage relaxation time calculated from the impedance plot hadhown a decreasing trend with increase in temperature. The opti-al band gap measured from the transmission spectra was foundo be 3.5 eV and a slight increase in the optical band gap of thelms with increase in substrate temperature was observed whichould be attributed to Burstein–Moss effect. From the magneticeasurements it is revealed that a paramagnetic like behavior is

bserved.

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